A gas turbine engine 10 is shown in FIG. 1 and comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32.
Gas turbine engines typically require numerous components to be clamped or bolted to other components. Such components could comprise bearings, stub shafts, discs or gears and these components may, for example, require retention onto a shaft or into a housing.
As shown in FIG. 2, a common method of retaining a component 8 to, for example, a shaft 4 or housing (not shown), is to use a threaded ring 2, which engages with a corresponding thread of the shaft 4. The threaded ring 2, component 8 and shaft 4 are each disposed about a centreline 6 of the shaft 4. In use, the ring 2 is threaded onto the shaft and rotated until the component 8 and ring 2 are in close contact with each other, and the ring 2 is then tightened onto the component 8 using tooling (not shown) which engages in slots formed between protrusions 11 which protrude from the ring 2. The component 8 is sandwiched between the threaded ring 2 and an abutment shoulder 13 in the shaft 4. The component is therefore secured to the shaft 4 by applying a clamp load in the direction A.
The clamp load generated by the threaded ring 2 between the shoulder 13 and the component 8 is determined by the amount of strain it introduces to the assembly comprising the shaft 4, the component 8 and the threaded ring 2. This in turn is determined by the amount the threaded ring is turned once all the components of the assembly are in close contact with each other.
The amount the threaded ring is turned is typically determined by applying a pre-calculated torque to the threaded ring 2. Another method used is angle-based tightening, in which the threaded ring 2 is turned through a predetermined angle once all the components of the assembly are in close contact with each other, regardless of the torque applied.
Where high clamp loads are required, high torques must be applied to the ring 2. During tightening however, friction occurs between the ring 2 and the shaft 4, and particularly between the threads of the ring 2 and shaft 4, and also between the ring 2 and the component 8. As a result, the torque that must be applied to the tool to turn the ring to the required extent is generally greatly in excess of the final torque that must be applied by the clamp ring in order to achieve the required clamp load. The torque that must be applied by the tool can be more than 10 and perhaps as much as 100 times as much the required final torque in the case of shafts having a diameter greater than 0.25 inches.
Such high torques can be difficult to apply. The high torques can also result in damage to one or both of the components, such as galling. Furthermore, variation in the coefficient of friction of the surfaces can result in large variations in the final torque for a given applied torque.
The present invention therefore seeks to address these issues.